Fine-scale variation in soil properties promotes local taxonomic diversity of hybridizing oak species (Quercus spp.)

Although many tree species frequently hybridize and backcross, management
decisions in forestry and nature conservation are usually concentrated on
pure species. Therefore, understanding which environmental factors drive
the distribution and admixture of tree species on local stand scale is of
great interest to support decision making in the establishment and
management of resilient forests. We extensively sampled a mixed stand of
hybridizing white oaks (Quercus petraea and Q. pubescens) near Lake
Neuchâtel (Switzerland), where limestone and glacier moraine geologies
coexist in proximity, to test whether micro-environmental conditions can
predict taxonomic distribution and genetic admixture. We collected DNA
from bud tissue, individual soil samples and extracted high-resolution
topographic data for 385 oak trees. We used 50 species-discriminatory
single nucleotide polymorphism (SNP) markers to determine the taxonomic
composition and admixture levels of individual trees and tested their
association with micro-environmental conditions. We show that the trees’
taxonomic distribution can be explained mainly by geographic position,
soil pH and potential rooting depth, a proxy for soil water availability.
We found that admixed individuals tend to grow in habitats being
characteristic for the more drought-tolerant species Q. pubescens rather
than in intermediate habitats. Using in-situ measurements, we are the
first to show that fine-scale variation in soil properties related to pH
and water availability potentially drives the distribution of hybridizing
tree species in a mixed stand. Micro-environmental variation therefore
promotes local taxonomic diversity, facilitates admixture and adaptive
introgression, and contributes to the resilience of forests under
environmental change. Consequently, species such as white oaks should be
managed and protected as a species complex rather than as pure species.